Information recording medium

ABSTRACT

The information recording medium of the invention comprises a substrate and a recording layer for recording information by a laser beam, and the recording layer contains a cyanine dye having the following formula (I): ##STR1## wherein each of A 1  and A 10  is an atom group for completing an aromatic ring which may have one or more substituents; L 1  is a methine group which may have one or more substituents or a trivalent connecting group of conjugated 3, 5 or 7 methines which may have one or more substituents; each of R 11 , R 12 , R 13  and R 14  is a hydrogen atom, or an alkyl, phenyl, acyl, alkoxy or 5- or 6-membered heterocyclic group which may have one or more substituents; each of R 15  and R 16  is an alkyl group which may have one or more substituents and one of said substituents may have an anionic moiety; X a   p-   is an anion; p is 1, 2 or 3, m is 1 or 0; and m is 0 when R 15  or R 16  has a substituent having an anionic moiety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an information recording medium havinga recording layer of a cyanine dye for recording information by means ofa laser beam having a high energy density and further relates to a novelcyanine dye employable for the information recording medium.

2. Description of Prior Art

An optical disc capable of recording information only once is referredto "DRAW" (Direct Read After Write), and studies on the optical dischave been made.

The optical disc of DRAW-type basically comprises a disc-shapedsubstrate made of a plastic material or a glass material and a recordinglayer made of a metal such as Bi, Sn, In and Te, a semi-metal or a dyesuch as a cyanine dye, a metal complex dye or a quinone dye, which isprovided on the substrate. On the surface of the substrate where therecording layer is to be provided, an intermediate layer may be providedto improve surface smoothness of the substrate and to enhance adhesionbetween the substrate and the recording layer or sensitivity of theresulting optical disc. Writing (i.e., recording) of information on theoptical disc can be carried out by irradiating the disc with a laserbeam. The irradiated area of the recording layer of the optical discabsorbs energy of the laser beam and rise of temperature locally occurs,and as a result, a chemical or physical change (e.g., formation of pits)is caused to alter (or change) optical characteristics of the recordinglayer in the irradiated area, whereby information is recorded on therecording layer. Reading (i.e., reproduction) of information from theoptical disc is also carried out by irradiating the disc with a laserbeam, that is, the recorded information is reproduced by detecting areflected light or a transmitted light corresponding to the change inthe optical characteristics of the recording layer.

As a material (i.e., recording material) for forming the recording layerof the information recording medium, metals and dyes (e.g., cyanine dye)are generally known, as described above. An information recording mediumusing a dye as a recording material has a merit in the characteristicsof an information recording medium, for example, the informationrecording medium has a higher sensitivity than the information recordingmedium using a metal as a recording material. In addition, the recordingmedium using a dye has such an advantageous feature in productivity thatthe recording layer can be easily formed on the substrate by aconventional coating method. For these reasons, dyes employable for suchoptical disc have been developed and put into practical use. Theproperties required for the dyes in application to the optical disc arefor example as follows: property of absorbing a light in the wavelengthregion of 700 to 900 nm, that is a wavelength region of a semiconductorlaser; relatively large absorption factor; and high reflectance.Recently, studies on enlarging of the wavelength region of asemiconductor laser or improvement and development of an optical discdrive have been made, and in accordance therewith, increase in number ofdyes employable for the optical disc and variation thereof are expected.Therefore, it is desired that various kinds of dyes employable for theoptical disc are developed.

The recording layers made of the above-mentioned dyes have such problemin optical characteristics that the layer generally has low reflectanceand low C/N of the reproduced signals. Further, such dye recording layereasily deteriorates with lapse of time owing to irradiation with alight.

Japanese Patent Provisional Publication No. 64(1989)-40382 discloses anoptical disc having a recording layer made of a cyanine dye havingbenzoindolenine structure as a dye recording layer improved in theabove-mentioned reflectance and C/N. An information recording mediumhaving such dye recording layer is relatively high in C/N, but isunsatisfactory in the reflectance and weathering resistance.

For enhancing a reflectance of the information recording medium, amethod of further providing a reflecting layer on the dye recordinglayer is generally made, and an example of the information recordingmedium having a reflecting layer on the dye recording layer is describedin "Nikkei Electronics", p. 107, Jan. 23, 1989. In the publication,although dyes used for the recording layer of the information recordingmedium are not apparent, recording of information on the recording layeris described as follows. That is, when the dye recording layer absorbs alaser beam to melt the dye of the layer, the plastic substrate is heatedto protrude on the dye recording layer side to form pits, so as torecord information on the recording layer. The reflecting layer providedin this information recording medium is a layer of Au-deposited film.According to the study of the present inventors, in the case that theabove-mentioned cyanine dye having benzoindolenine structure is used forthe dye recording layer, an optical disc having a high C/N and improvedreflectance is obtained.

In the optical disc of DRAW-type, high-density recording of CD formatsignals (i.e., CD-DRAW) is necessarily made at a low linear speed of 1.2to 1.4 m/sec, and the recorded signals are required to be reproducedusing commercially available CD players. In the case of reproducing therecorded signals using the CD players, the optical disc is desired tohave a reflectance of not less than 70%. However, even if CD formatsignals are recorded on the optical disc having a recording layer madeof a cyanine dye having the above-mentioned benzoindolenine structureand a reflecting layer on the recording layer, some of the commerciallyavailable CD players cannot reproduce the recorded signals.

Accordingly, it is desired to produce optical discs having a markedlyhigh reflectance (approx. 80%) which are employable as theabove-mentioned CD-DRAW type discs.

SUMMERY OF THE INVENTION

It is an object of the invention to provide an information recordingmedium having a recording layer made of a cyanine dye.

It is an another object of the invention to provide an informationrecording medium which has a recording layer made of a cyanine dye andis prominently improved in C/N and reflectance.

It is a further object of the invention to provide an informationrecording medium which has a recording layer made of a cyanine dye andshows excellent weathering resistance in addition to prominentlyenhanced C/N and reflectance.

It is a still further object of the present invention to provide a novelcyanine dye (polymethine compound) employable for an informationrecording medium.

There is provided by the present invention an information recordingmedium comprising a substrate and a recording layer for recordinginformation by means of a laser beam which is provided on the substrate,in which the recording layer contains a cyanine dye having the formula(I): ##STR2## wherein each of A¹ and A¹⁰ is an atom group for completingan aromatic ring which may have one or more substituents; L¹ is amethine group which may have one or more substituents or a trivalentconnecting group of conjugated 3, 5 or 7 methines which may have one ormore substituents; each of R¹¹, R¹², R¹³ and R¹⁴ is a hydrogen atom, oran alkyl, phenyl, acyl, alkoxy or 5- or 6-membered heterocyclic groupwhich may have one or more substituents; each of R¹⁵ and R¹⁶ is an alkylgroup which may have one or more substituents and one of saidsubstituents may have an anionic moiety; X_(a) ^(p-) is an anion; p is1, 2 or 3; m is 1 or 0; and m is 0 when R¹⁵ or R¹⁶ has a substituenthaving an anionic moiety.

Preferred embodiments of the information recording medium of the presentinvention are as follows.

1) The information recording medium wherein each of A¹ and A¹⁰ in theabove formula (I) is an atom group for completing a benzene ornaphthalene ring which may be substituted with a halogen atom, ahaloganated alkyl group of 1-4 carbon atoms, a cyano group or analkoxycarbonyl group of 1-4 carbon atoms.

2) The information recording medium wherein L¹ in the above formula (I)is a trivalent connecting group of 3 or 5 methines which may besubstituted with a halogen atom, an alkyl group of 1-4 carbon atoms (thealkyl group may be bonded to other methine group to form a ring), abenzyl group or a phenyl group.

3) The information recording medium wherein each of R¹¹, R¹², R¹³ andR¹⁴ in the above formula (I) is an unsubstituted alkyl group of 1-8carbon atoms, an unsubstituted phenyl group, or a naphthyl group.

4) The information recording medium wherein each of R¹⁵ and R¹⁶ in theabove formula (I) is an alkyl group of 1-18 carbon atoms which may besubstituted with an alkoxy group of 1-4 carbon atoms, an alkylthio groupof 1-4 carbon atoms, an alkoxycarbonyl group of 1-4 carbon atoms, asulfonyl group of 1-4 carbon atoms, a halogen atom or a hydroxy group.

5) The information recording medium wherein X_(a) ^(p-) in the aboveformula (I) is a halogen ion, a sulfonate ion, ClO₄ ⁻, BF₄ ⁻, a metalcomplex ion, or a phosphoric acid ion.

6) The information recording medium wherein wherein each of A¹ and A¹⁰,R¹¹ and R¹³, R¹² and R¹⁴, and R¹⁵ and R¹⁶ in the formula (I) of thecyanine dye is the same as each other, A¹ and A¹⁰ are atom groups forcompleting a benzene ring which has one or more substituents or anaphthalene ring which may have one or more substituents, and R¹¹, R¹³,R¹² and R¹⁴ are hydrogen atoms, alkyl groups which may have one or moresubstituents or phenyl groups which may have one or more substituents.This cyanine dye can be represented by the formula (II) as describedbelow.

7) The information recording medium wherein a reflecting layer made of ametal is provided on the recording layer.

8) The information recording medium wherein the recording layer is madeof a mixture of a cyanine dye having the above formula (I) and a dyehaving absorption maximum on the longer wavelength side than theabsorption maximum of the.

9) The information recording medium wherein the recording layer furthercontains a quencher having absorption maximum on the longer wavelengthside than the absorption maximum of any dye contained in the recordinglayer.

10) The information recording medium as described in the aboveembodiment 8) wherein the absorption maximum of the dye is longer thanthe absorption maximum of the cyanine dye having the above formula (I)by not less than 20 nm.

11) The information recording medium wherein the absorption maximum ofthe cyanine dye is located on the longer wavelength side than 650 nm.

12) The information recording medium wherein a material of the substrateis plastic.

13) The information recording medium as described in the aboveembodiment 7) wherein the metal of the reflecting layer is selected fromthe group consisting of Cr, Ni, Pt, Cu, Ag, Au, Al and a stainlesssteel.

The terms "absorption maximum of a dye" means absorption maximum of adye recording layer formed on the substrate.

As described above, the preferred cyanine dye has the formula (II):##STR3## wherein A² is an atom group for completing a benzene ring whichhas one or more substituents or a naphthalene ring which may have one ormore substituents; L² is a methine group which may have one or moresubstituents or a trivalent connecting group of conjugated 3, 5 or 7methines which may have one or more substituents; each of R²¹ and R²² isa hydrogen atom, an alkyl group which may have one or more substituentsor a phenyl group which may have one or more substituents; R²³ is analkyl group which may have one or more substituents and one of saidsubstituents may have an anionic moiety; X_(b) ^(n-) is an anion; n is1, 2 or 3; j is 1 or 0; and j is 0 when R²³ has a substituent having ananionic moiety.

The cyanine dyes of the formula (II) are new.

Preferred embodiments of the cyanine dye of the formula (II) are asfollows.

1) The cyanine dye having the above formula (II) wherein A² is an atomgroup for completing a benzene or naphthalene ring substituted with ahalogen atom, a cyano group, a nitro group, a carboxyl group, a sulfonicacid group, an acyl group, an alkylsulfonyl group of 1-8 carbon atoms,an arylsulfonyl group of 1-8 carbon atoms, an alkoxycarbonyl group of1-8 carbon atoms, a sulfamoyl group of 1-8 carbon atoms, anN-substituted sulfamoyl group of 1-8 carbon atoms, a carbamoyl group of1-8 carbon atoms, an N-substituted carbamoyl group of 1-8 carbon atoms,an acylamino group of 1-8 carbon atoms, a ureido group of 1-8 carbonatoms, an amide group of 1-8 carbon atoms, an alkyl group of 1-8 carbonatoms, an aryl group of 6-8 carbon atoms, an alkoxy group of 1-8 carbonatoms, an aryloxy group of 6-8 carbon atoms, an alkylthio group of 1-8carbon atoms, an arylthio group of 6-8 carbon atoms, a fluoroalkyl groupof 1-8 carbon atoms, 5-membered or 6-membered heterocyclic ring, or agroup containing a 5-membered or 6-membered heterocyclic ring, or anatom group for completing an unsubstituted naphthalene ring.

2) The cyanine dye having the above formula (II) wherein L² is atrivalent connecting group of 3 or 5 methines which may be substitutedwith a halogen atom, an alkyl group of 1-4 carbon atoms (the alkyl groupmay be bonded to other methine group to form a ring), a benzyl group ora phenyl group.

3) The cyanine dye having the above formula (II) wherein each of R²¹ andR²² is an unsubstituted alkyl group of 1-8 carbon atoms, anunsubstituted phenyl group, or a phenyl group substituted with an alkoxygroup.

4) The cyanine dye having the above formula (II) wherein R²³ is an alkylgroup of 1-18 carbon atoms which may be substituted with an alkoxy groupof 1-4 carbon atoms, an alkylthio group of 1-4 carbon atoms, analkoxycarbonyl group of 1-4 carbon atoms, a sulfonyl group, a halogenatom, or a hydroxyl group.

5) The cyanine dye having the above formula (II) wherein X_(b) ^(n-) isa halogen ion, a sulfonate ion, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, an acetic acidion, a metal complex ion, or a phosphoric acid ion.

6) The cyanine dye having the above formula (II) wherein A² is an atomgroup for completing a benzene or naphthalene ring substituted with ahalogen atom, a cyano group, a nitro group, a carboxyl group, a sulfonicacid group, an acyl group of 1-4 carbon atoms, an arylsulfonyl group of6-8 carbon atoms, an alkoxycarbonyl group of 1-4 carbon atoms, aN-substituted sulfamoyl group of 1-4 carbon atoms, a ureido group of 1-4carbon atoms, an amide group of 1-4 carbon atoms, an alkyl group of 1-4carbon atoms, an aryl group of 6-8 carbon atoms, an alkoxy group of 1-4carbon atoms an aryloxy group of 6-8 carbon atoms, an alkylthio group of1-4 carbon atoms, an arylthio group of 6-8 carbon atoms, a fluoroalkylgroup of 1-4 carbon atoms, a morphorinocarbonyl group, or amethylenedioxy group, or an atom group for completing an unsubstitutednaphthalene ring.

7) The cyanine dye having the above formula (II) wherein L² is atrivalent connecting group of 3 or 5 methines which may be substitutedwith a halogen atom, an alkyl group of 1-4 carbon atoms which may bebonded to other methine group to form a ring, a benzyl group or a phenylgroup.

8) The cyanine dye having the above formula (II) wherein each of R²¹ andR²² is an unsubstituted alkyl group of 1-3 carbon atoms, anunsubstituted phenyl group, or a phenyl group substituted with an alkoxygroup of 1-3 carbon atoms.

9) The cyanine dye having the above formula (II) wherein R²³ is an alkylgroup of 1-4 carbon atoms which may be substituted with an alkoxy groupof 1-4 carbon atoms, an alkylthio group of 1-4 carbon atoms, analkoxycarbonyl group of 1-4 carbon atoms, a sulfonyl group, a fluorineatom, or a hydroxy group.

10) The cyanine dye having the above formula (II) wherein X_(b) ^(n-) isa halogen ion, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, an acetate ion, ap-toluenesulfonate ion or CF₃ SO₃ ⁻.

The optical disc having a recording layer made of a specific cyanine dyerepresented by the above formula (I), particularly the specific cyaninedye including the cyanine dye having the above formula (II), showsexcellent recording and reproducing characteristics and highreflectance. Especially in the case of providing a recording layer madeof the novel cyanine dye having the formula (II) on the substrate, aprominently high reflectance can be obtained.

Moreover, in the case of providing a recording layer made of the cyaninedye of the invention on the substrate and further laminating areflecting layer on the recording layer, a reflectance can be improvedwith little lowering in recording and reproducing characteristics suchas recording sensitivity, C/N and modulation degree.

In particular, when a recording layer made of a mixture of the specificcyanine dye of the invention and other dye having absorption maximum onthe longer wavelength side than the absorption maximum of the cyaninedye is provided on the substrate and a reflecting layer made of a metalis further provided on the recording layer, recording and reproducingcharacteristics such as recording sensitivity, C/N and modulation degreeare enhanced in addition to the improvement of reflectance.

Furthermore, the optical disc of the invention shows a prominently highreflectance, so that CD format signals recorded on the optical disc canbe reproduced using commercially available CD players, and hence theoptical disc of the invention is very useful as CD-DRAW.

The novel cyanine dye having the above formula (II) has a substituent(including unsubstituted naphthyl group) in an aromatic ring of abenzimidazole ring which is positioned at each end of the polymethinechain. Therefore, there can be selected a cyanine dye having desiredcharacteristics such as solubility and absorption spectrum from thevarious cyanine dyes (of the formula (II)). Hence, the cyanine dye ofthe present invention can be utilized for markedly wide applicationssuch as optical filters, dyes and pharmaceuticals as well as the aboveinformation recording media.

DETAILED DESCRIPTION OF THE INVENTION

The information recording medium of the present invention basicallycomprises a substrate and a recording layer provided thereon whichcontains a cyanine dye having the aforementioned formula (I).

One embodiment of the information recording medium of the invention hasa basic structure wherein a reflecting layer is further provided on theabove-mentioned dye recording layer.

The present inventors have earnestly studied for providing novel dyescapable of forming a recording layer on the substrate by means of acoating method and showing both of high reflectance and excellentrecording and reproducing characteristics such as high C/N when used foroptical discs.

As a result, they have found that the above-mentioned excellentcharacteristics can be obtained by using a cyanine dye having theaforementioned formula (I) for the recording layer. Further, they havealso found that the reflectance can be improved with little reduction ofthe recording and reproducing characteristics such as recordingsensitivity, C/N and modulation degree by superposing a reflecting layermade of a metal on the recording layer made of the above-mentionedspecific cyanine dye. Especially in the case of providing a recordinglayer made of a novel cyanine dye having the formula (II) of thespecific structure among the compounds having the formula (I), aprominently enhanced reflectance can be obtained.

Moreover, the recording and reproducing characteristics such asrecording sensitivity, C/N and modulation degree can be easily enhancedin addition to the high reflectance by providing a recording layer madeof a mixture of a cyanine dye having the formula (I) and other dye whoseabsorption maximum is positioned on the longer wavelength side than theabsorption maximum of the cyanine dye.

That is, in the cyanine dye having the formula (I) of the invention, theabsorption maximum can be varied in its position to locate on either theshorter wavelength side or the longer wavelength side by changing thestructure thereof. Most of the above-mentioned cyanine dyes have suchproperties that they show high reflectance and relatively high C/N andmodulation degree within the wavelength region of approx. 780 nm, thatis a wavelength of ordinary laser beam. By using a dye of the inventionhaving absorption maximum within the wavelength region shorter than 780nm among the above-mentioned compounds of the invention and a dyeshowing high sensitivity within the wavelength region of the laser beam(i.e., other dye whose absorption maximum is positioned on the longerwavelength side the cyanine dye) in combination, recording andreproducing characteristics such as recording sensitivity, C/N andmodulation degree is enhanced in addition to the high reflectance.

The information recording medium of the invention can be prepared, forexample, by the following process.

A material of the substrate employable in the invention can be selectedfrom any materials which have been employed for substrates of theconventional information recording media. Examples of the substratematerials employable in the invention include glasses, acrylic resins(e.g., polymethyl methacrylate), vinyl chloride resins (e.g., polyvinylchloride and vinyl chloride copolymer), epoxy resins, amorphouspolyolefins and polyesters. These materials may be employed incombination, if desired. Further, these materials can be employed in theform of a film, or can be employed as a rigid substrate. Of thesematerials, preferred is polycarbonate from the viewpoints of moistureresistance, dimensional stability, manufacturing cost, etc.

On the surface of the substrate where the recording layer is to beprovided, an undercoat layer may be provided to improve surfacesmoothness of the substrate, to enhance the adhesion between thesubstrate and the recording layer and to prevent denaturing of therecording layer. Examples of materials for the undercoat layer includepolymer materials such as polymethyl methacrylate, acrylic acidmethacrylic acid copolymer, styrene/maleic anhydride copolymer,polyvinyl alcohol, N-methylol acrylamide, styrene/vinyl toluenecopolymer, chlorosulfonated polyethylene, nitrocellulose, polyvinylchloride, chlorinated polyolefin, polyester, polyimide, vinylacetate/vinyl chloride copolymer, ethylene/vinyl acetate copolymer,polyethylene, polypropylene and polycarbonate; and organic materialssuch as silane coupling agent.

The undercoat layer can be formed on the substrate, for example, bydissolving or dispersing the above-mentioned material in an appropriatesolvent to prepare a coating solution or dispersion, applying thecoating solution or dispersion to the surface of the substrate usingknown coating methods such as spin coating, dip coating and extrusioncoating. The thickness of the undercoat layer is generally in the rangeof 0.005 to 20 μm, preferably in the range of 0.01 to 10 μm.

On the substrate (or undercoat layer), tracking grooves or protruded anddepressed portions (pits) indicating address signals are preferablyformed. In the case of using a resin material (e.g., polycarbonate) asthe substrate material, it is preferred to directly form the grooves onthe substrate by subjecting the resin material to injection molding,extrusion molding or the like.

Formation of the grooves can be also made by providing a pre-groovelayer on the substrate. As a material of the pre-groove layer, there canbe employed a mixture of at least one monomer (or oligomer) selectedfrom the group consisting of monoester, diester, triester and tetraesterof an acrylic acid and a photopolymerization initiator.

The pre-groove layer can be formed on the substrate by the processdescribed below. In the first place, a mixture liquid of an acrylic acidester and a photopolymerization initiator is coated on a preciselyprepared stamper, and on the coated layer of the liquid is placed asubstrate. Then, the coated layer is cured under irradiation withultraviolet rays via the stamper or the substrate so as to fix thesubstrate to the coated layer. Thereafter, the substrate is separatedfrom the stamper. Thus, a substrate provided with a pre-groove layer canbe prepared.

The thickness of the pre-groove layer is generally in the range of 0.05to 100 μm, preferably in the range of 0.1 to 50 μm.

On the substrate, a recording layer is provided.

The recording layer of the information recording medium according to theinvention is characterized in that the layer contains a cyanine dyehaving the following formula (I): ##STR4## wherein each of A¹ and A¹⁰ isan atom group for completing an aromatic ring which may have one or moresubstituents; L¹ is a methine group which may have one or moresubstituents or a trivalent connecting group of conjugated 3, 5 or 7methines which may have one or more substituents; each of R¹¹, R¹², R¹³and R¹⁴ is a hydrogen atom, or an alkyl, phenyl, acyl, alkoxy or 5- or6-membered heterocyclic group which may have one or more substituents;each of R¹⁵ and R¹⁶ is an alkyl group which may have one or moresubstituents and one of said substituents may have an anionic moiety;X_(a) ^(p-) is an anion; p is 1, 2 or 3; m is 1 or 0; and m is 0 whenR¹⁵ or R¹⁶ has a substituent having an anionic moiety.

The atom group indicated by A¹ or A¹⁰ for completing an aromatic ringpreferably is a benzene ring which is substituted or a naphthalene ringwhich may be substituted. Examples of substituents on the benzene ringor naphthalene ring indicated by A¹ or A¹⁰ include a halogen atom (e.g.,F or Cl), a cyano group, a nitro group, a carboxyl group, a sulfonicacid group, an acyl group (e.g., acetyl or benzoyl), an alkylsulfonylgroup (e.g., methanesulfonyl or butanesulfonyl), an arylsulfonyl group(e.g., phenylsulfonyl and 3-chlorophenylsulfonyl), an alkoxycarbonylgroup (e.g., ethoxycarbonyl or 2-ethoxycarbonyl), a sulfamoyl group oran N-substituted sulfamoyl group (e.g., butylsulfamoyl, phenylsulfamoylor dibutylsulfamoyl), a carbamoyl group or an N-substituted carbamoylgroup (e.g., ethylcarbamoyl or phenylcarbamoyl), an acylamino group(e.g., acetylamino or benzoylamino), a ureido group (e.g., methylureido, ethyl ureido or phenyl ureido), an amide group (e.g.,methylamide), an alkyl group (e.g., methyl, ethyl or isobutyl), an arylgroup (e.g., benzyl), an alkoxy group (e.g., methoxy, butoxy or2-methoxyethoxy), an aryloxy group (e.g., phenoxy or 4-chlorophenoxy),an alkylthio group (e.g., methylthio, butylthio or benzylthio), anarylthio group (e.g., phenylthio or 4-butylphenylthio), a 5-membered or6-membered heterocyclic ring, and a group containing a heterocyclic ring(e.g., morpholinocarbonyl). In these substituents, each group preferablyhas 1-8 carbon atoms.

Particularly preferred is a substituent of which Hammet σ constant ispositive or conversion value of Hammet σ constant is positive. Mostpreferred are a halogen atom (e.g., F or Cl), a halogenated alkyl groupof 1-4 carbon atoms (e.g., trifluoromethyl), a cyano group and analkoxycarbonyl group of 1-4 carbon atoms (e.g., ethoxycarbonyl).

A preferred example of the connecting group indicated by L¹ is aconnecting group of 3 or 5 methines which may have one or moresubstituents. Preferred substituents are a halogen atom such as F andCl, an alkyl group of 1-8 carbon atoms, an alkoxy group of 1-6 carbonatoms, an aralkyl group and an aryl group of 6-8 carbon atoms.Particularly preferred are a halogen atom, an alkyl group of 1-4 carbonatoms, a benzyl group and a phenyl group. Most preferred are methyl,benzyl and a halogen atom such as Cl.

As the group indicated by R¹¹, R¹², R¹³ or R¹⁴, there can be mentionedan alkyl group of 1-18 (preferably 1-4) carbon atoms which may besubstituted (e.g., methyl, ethyl, butyl, isobutyl, 2-ethylhexyl,dodecyl, trifluoromethyl, 2-ethoxyethyl, 2-hydroxyethyl, 3-sulfopropyl,3-sulfobutyl or 2-sulfoethyl), and a phenyl or naphthyl group of 6-18(preferably 6-10) carbon atoms which may be substituted (e.g., phenyl,4-methylphenyl, 3,5-dichlorophenyl, 4-carboxyphenyl, 4-methoxyphenyl,β-naphthyl or 2,5-di-tert-amylphenyl). Further, as the group of R¹¹,R¹², R¹³ or R¹⁴, there can be mentioned an acyl group such as acetyl andbenzoyl, an alkoxy group such as methoxy and phenoxy, and a 5- or6-membered heterocyclic group such as 2-pyridyl and 4-pyridyl.

Particularly, preferred are an unsubstituted alkyl group of 1-8 carbonatoms, an unsubstituted or alkoxysubstituted phenyl group and anunsubstituted naphthyl group, and most preferred are methyl, ethyl,n-propyl and phenyl.

As the group indicated by R¹⁵ or R¹⁶, there is preferably mentioned analkyl group of 1-18 (preferably 1-5) carbon atoms which may besubstituted, for example, an unsubstituted alkyl group (e.g., methyl,ethyl, isopropyl, sec-butyl, cyclohexyl or 2-ethylhexyl); an alkyl groupsubstituted with a non-acidic group such as an alkoxy group of 1-4carbon atoms, an alkylthio group of 1-4 carbon atoms, an alkoxycarbonylgroup of 1-4 carbon atoms, a halogen atom or a hydroxyl group (e.g.,2-ethoxyethyl, 2-methylthiopropyl, benzyl, 3-phenylpropyl, dodecyl or3-acetoxypropyl); and an alkyl group of 1-4 carbon atoms which issubstituted with a sulfo or carboxyl group (e.g., 3-sulfopropyl,3-sulfobutyl, 4-sulfobutyl, 2-sulfoethyl or 3-caboxypropyl). When R¹⁵ orR¹⁶ is a substituent having an anionic moiety, the substituent forms aninner salt. Further, when R¹⁵ and R¹⁶ are substituents having an anionicmoiety, it is possible that one is a salt with a metal or the like andthe other forms an inner salt.

There is more preferably mentioned methyl, ethyl, isopropyl, sec-butyl,cyclohexyl, 2-ethylhexyl, benzyl, 2-phenylethyl, dodecyl, 2-ethoxyethyl,3-ethoxypropyl, hydroxyethyl, 3-acetoxypropyl, ethoxycarbonylmethyl,3-sulfopropyl, 2,3-tetrafluoropropyl, allyl, 2-methylthioethyl and3-methoxypropyl. Most preferred are methyl, ethyl, sec-butyl,2-ethoxyethyl, 3-ethoxypropyl, 3-methoxypropyl, hydroxyethyl,3-acetoxypropyl, ethoxycarbonylmethyl, 3-sulfopropyl and2,3-tetrafluoropyrpyl.

Preferred examples of the anion indicated by R_(a) ^(p-) are a halideion (e.g., Cl⁻, Br⁻ and I⁻), a sulfonate ion (e.g., CH₃ SO₃ ⁻, CF₃ SO₃⁻, CH₃ OSO₃ ⁻, ##STR5## a naphthalene-1,5-disulfonate ion), ClO₄ ⁻, BF₄⁻, a metal complex ion (e.g., ##STR6## and a phosphoric acid ion (e.g.,PF₆ ⁻, H₂ PO₄ ⁻, t-C₄ H₉ ##STR7## Particularly preferred are ClO₄ ⁻, PF₆⁻ and ##STR8##

In these anions, I⁻ used in the intermediate stage of the synthesis or##STR9## may be contained in an extremely small amount.

P preferably is 1 or 2.

In the compound having the formula (I) employable for the informationrecording medium of the invention, R¹¹ and R¹³, R¹² and R¹⁴, R¹⁵ andR¹⁶, and A¹ and A¹⁰ are each preferably the same as each other, A¹ andA¹⁰ are benzene rings which has a substituent or naphthalene rings whichmay have one or more substituents, and R¹¹, R¹³, R¹² and R¹⁴ arehydrogen atoms, alkyl groups which may have one or more substituents orphenyl groups which may have one or more substituents. That is, thepreferred formula is the following formula (II): ##STR10## wherein A² isan atom group for completing a benzene ring which has one or moresubstituents or a naphthalene ring which may have one or moresubstituents; L² is a methine group which may have one or moresubstituents or a trivalent connecting group of conjugated 3, 5 or 7methines which may have one or more substituents; each of R²¹ and R²² isa hydrogen atom, an alkyl group which may have one or more substituentsor a phenyl group which may have one or more substituents; R²³ is analkyl group which may have one or more substituents and one of saidsubstituents may have an anionic moiety; X_(b) ^(n-) is an anion; n is1, 2 or 3; j is 1 or 0; and j is 0 when R²³ has a substituent having ananionic moiety.

Examples of substituents on the benzene ring or naphthalene ringindicated by A² are those described for A¹ and A¹⁰ in the aforementionedformula (I).

The connecting group indicated by L² is preferably a connecting group of2, 3 or 5 methines which may have one or more substituents. Preferredexamples of the substituents are those described for L¹ in theaforementioned formula (I).

Preferred examples of the group indicated by R²¹ and R²² are thosedescribed as examples of the alkyl, phenyl or naphthyl group indicatedby R¹¹, R¹², R¹³ or R¹⁴ in the aforementioned formula (I).

As examples of the group indicated by R²³, there can be mentioned thosedescribed as examples of the group indicated by R¹⁵ or R¹⁶ in theaforementioned formula (I).

As preferred examples of the anion indicated by X_(b) ^(n-), there canbe mentioned those described as examples of the anion indicated by X_(a)^(p-) in the aforementioned formula (I).

n is preferably 1 or 2.

Concrete examples of the compounds having the above formula (I) aredescribed below (I-1 to I-47). The compounds of I-1 to I-43 are includedin the cyanine dyes of the formula (II). ##STR11##

The cyanine dyes (polymethine compounds) of the formula (I) having nosubstituent on the benzene ring of the benzimidazole ring are describedin a report "Khim. Geterotsikl. Soedin.", 1967, No. 5, pp. 917-922, (inRussian), written by Babichev and Babicheva. See "Chemical Abstracts",Vol. 69, p. 5,626, Sec. 60031V. Examples of the synthesis processes arehereinafter described in Examples 1-2.

As described hereinbefore, in the case that the above-mentioned dye andother dye showing high sensitivity at the wavelength of a laser beam(i.e., dye having absorption maximum on the longer wavelength side thanthe absorption maximum of the above-mentioned dye) are used incombination, there can be obtained an information recording mediumimproved in the recording and reproducing characteristics such asrecording sensitivity, C/N and modulation degree in addition to the highreflectance. Employment of such combination of dyes as a dye componentof the recording layer is very effective especially in the case ofproviding a reflecting layer of a metal such as Au on the recordinglayer.

In the optical disc having a reflecting layer on the recording layer,when the cyanine dye having the above formula (I) is used in combinationwith other dye having absorption maximum on the longer wavelength sidethan the absorption maximum of the cyanine dye, the absorption maximumof the dye used in combination with the cyanine dye preferably is higherthan the absorption maximum of the cyanine dye by not less than 20 nmfrom the viewpoint of enhancement of C/N and reflectance.

As the dyes used in combination with the cyanine dye, any dyes can beemployed, provided that they satisfy the above-mentioned conditions.However, in order to form a uniform dye recording layer, preferablyemployed are those having high compatibility with the dye of theaforementioned formula (I) and being soluble in the same organic solventas that for the dye of the formula (I). By forming a uniform dyerecording layer, the resulting information recording medium comes toexhibit excellent recording and reproducing characteristics such as highC/N and high modulation degree. Preferred examples of such dyes arepolymethine dyes such as cyanine dye, oxonol dye, pyrylium dye andthiopyrylium dye.

Particularly in the case of an optical disc having a reflecting layer onthe recording layer and when the recording layer is formed using amixture of a specific cyanine dye having benzoindolenine structurerepresented by the following formula (III) that has a relatively highreflectance and also is excellent in the recording and reproducingcharacteristics such as C/N and recording sensitivity and a dye of theformula (I) of the invention, the above-mentioned recording andreproducing characteristics and the reflectance can be prominentlyenhanced.

Accordingly, as the dyes having absorption maximum on the longerwavelength side, preferred are cyanine dyes having benzoindoleninestructure which are represented by the following formula (III):##STR12## wherein each of R³¹, R³² and R³³ is an alkyl group of 1-8carbon atoms which may have one or more substituents; X^(q-) is ananion; and q is 1 or 2.

As the alkyl group of 1-8 carbon atoms which may have one or moresubstituents, that is indicated by R³¹ in the formula (III), there canbe mentioned methyl, ethyl, n-propyl, n-butyl, isobutyl and2-ethylhexyl. Preferred is an alkyl group of 1-6 carbon atoms (e.g.,methyl, ethyl, n-propyl, n-butyl or isobutyl). Examples of thesubstituents of the alkyl group are a fluorine atom and an alkoxy group.Particularly preferred is an unsubstituted alkyl group.

As the alkyl group indicated by R³² or R³³, preferred is anunsubstituted alkyl group having 1-8 carbon atoms (e.g., methyl, ethylor propyl), and particularly preferred are methyl and ethyl.

X^(q-) is the same anion as indicated by X^(p-) in the aforementionedformula (I).

Examples of the compounds having the formula (III) include the followingcompounds III-1 to III-16: ##STR13##

The cyanine dyes having the indolenine structure (represented by theabove formula (III)) which are exemplified as above are described in theaforementioned Japanese Patent Provisional Publications No.64(1989)-40382 and No. (1989)-40387.

In a combination of the dye having the formula (I) and the dye havingthe formula (III), L¹ in the formula (I) is preferably composed of 3methines (the methines may be substituted) bonded to each other, and themixing ratio between the dye having the formula (I) and the dye havingthe formula (III) preferably is in the range of 9:91 to 80:20 (dyehaving the formula (I):dye having the formula (III), by weight).

In order to enhance weathering resistance, it is preferred to usevarious dyes known as oxygen quenchers of singlet state, for example,compounds having the following formula (IV) or (V), in combination withthe above-mentioned dye(s): ##STR14## wherein [Cat]⁺ is a non-metalliccation such as tetraalkyl nonium; M is a transition metal atom such asNi; and each of Z and Z' is an atom group for completing a 5-memberedheterocyclic ring or 6-membered aromatic ring such as2-thioxo-1,3-dithiol ring or a benzene ring which may be substituted.##STR15##

wherein R is an alkyl group which may have one or more substituents; andQ is the same anion as indicated by X_(a) ^(p-) in the formula (I).

Examples of the quencher represented by the above formula (IV) or (V)include PA-1006 (trade name, available from Mitsui Toatsu Fine Co.,Ltd.) and IRG-023 (trade name, available from Nippon Kayaku Co., Ltd.).

Formation of the recording layer can be conducted by dissolving theabove-mentioned dye (and the above-mentioned quencher, binder, etc., ifdesired) in a solvent to prepare a coating solution, coating thesolution over the surface of the substrate, and drying the coated layerof the solution.

Examples of the solvents employable for preparing the coating solutionfor the formation of the dye recording layer include esters such asethyl acetate, butyl acetate and cellosolve acetate; ketones such asmethyl ethyl ketone, cyclohexanone and methyl isobutyl ketone;chlorinated hydrocarbons such as dichloromethane, 1,2-dichloroethane andchloroform; amides such as dimethylformamide; hydrocarbons such ascyclohexane; ethers such as tetrahydrofuran, ethyl ether and dioxane;alcohols such as ethanol, n-propanol, isopropanol and n-butanol; andfluorine-containing solvents such as 2,2,3,3-tetrafluoropropanol.

The coating solution may further contain various additives such as anantioxidant, a UV-absorbent, a plasticizer and a lubricant according tothe purpose.

A binder may be contained in the coating solution. Examples of thebinders include natural organic polymer materials such as gelatin,cellulose derivative, dextran, rosin and rubber; and synthetic organicpolymer materials such as hydrocarbon resins (e.g., polyethylene,polypropylene, polystyrene, polyisobutylene and chlorinatedpolyethylene), vinyl resins (e.g., polyvinyl chloride, polyvinylidenechloride and polyvinyl chloride/polyvinyl acetate copolymer, polyvinylalcohol, butyral resin), acrylic resins (e.g., polymethyl acrylate andpolymethyl methacrylate), and precondensates of thermosetting resins(e.g., epoxy resin, rubber derivative and phenol/formaldehyde resin).

In the case of using a binder for the formation of the recording layer,a ratio of the dye to the binder is generally in the range of 0.01 to99% by weight, preferably in the range of 1.0 to 95% by weight. Thecoating solution prepared as above generally has a concentration of thedye in the range of 0.01 to 10% by weight, preferably 0.1 to 5% byweight.

The recording layer may have either single-layer structure ormulti-layer structure, and in any case, the thickness of the recordinglayer is in the range of 200 to 3,000 Å, preferably 500 to 2,500 Å.Further, the recording layer may be provided on one surface of thesubstrate or both surfaces of the substrate.

The coating solution can be coated on the substrate utilizingconventional coating methods such as spray coating, spin coating, dipcoating, roll coating, blade coating, doctor roll coating and screenprinting.

In the information recording medium of the invention, a reflecting layeris preferably provided on the recording layer for the purpose ofenhancing the reflectance in the information-reproduction stage.

A light-reflecting material employable for the reflecting layer is amaterial having a high reflectance to a laser beam. Examples of thelight-reflecting materials include metals and semi-metals such as Mg,Se, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Ru, Rh, Pd,Ir, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Si, Ge, Te, Pb, Po, Sn and Bi;and stainless steel. Cr, Ni, Pt, Cu, Ag, Au, Al and stainless steel arepreferred. These materials can be employed singly or in combination.Alloys thereof can be also employed.

The reflecting layer can be formed on the recording layer using theabove-mentioned light-reflecting material according to a known methodsuch as deposition, sputtering or ion plating. The thickness of thereflecting layer is generally in the range of 100 to 3,000 Å.

On the reflecting layer, a protective layer may be provided tophysically and chemically protect the recording layer. The protectivelayer may be provided also on the surface of the substrate where therecording layer is not provided for the purpose of improving physicalresistance and moisture resistance.

As materials of the protective layer, there can be mentioned inorganicmaterials such as SiO, SiO₂, MgF₂, SnO₂ and Si₃ N₄ ; and organicmaterials such as thermoplastic resins, thermosetting resins andUV-curable resins.

The protective layer can be formed, for example, by laminating a plasticfilm having been obtained by extrusion processing of a plastic materialon the recording layer (or a reflecting layer) and/or the substrate viaan adhesive layer. Otherwise, the protective layer can be providedthereon utilizing vacuum deposition, sputtering, coating, etc. In thecase of using a thermoplastic resin or a thermosetting resin as amaterial of the protective layer, the resin is dissolved in anappropriate solvent to prepare a coating solution, the coating solutionis coated over the recording layer and/or the substrate, and the coatedlayer of the solution is then dried to form a protective layer. In thecase of using a UV-curable resin, the resin per se or a solution of theresin in an appropriate solvent is coated over the recording layerand/or the substrate, and the coated layer of the solution is irradiatedwith ultraviolet rays to cure the layer so as to form a protectivelayer. The coating solution for the formation of a protective layer mayfurther contain a variety of additives such as an antistatic agent, anantioxidant and a UV-absorbent according to the purpose.

The thickness of the protective layer is generally in the range of 0.1to 100 μm.

The information recording medium of the invention may be a single platetype having the above-described structure, or may be a combined-type inwhich two substrates each having the above-described structure arecombined using an adhesive in such a manner that the recording layers onthe substrates are positioned inside to face each other. Otherwise, aninformation recording medium of air-sandwich type can be prepared byusing two of disc-shaped substrates, at least one of them having theabove-described structure, and combining them by way of a ring-shapedinner spacer and a ring-shaped outer spacer.

The optical disc obtained as above and having a markedly highreflectance and excellent recording and reproducing characteristics cankeep desired reflectance, even if a quencher of singlet state which isgenerally used for improving resistance to light is added to the opticaldisc. When the quencher is added to a conventional optical disc, thereflectance of the optical disc generally decreases. However, thereflectance of the information recording medium according to theinvention is prominently high, namely, approx. 80%, so that even if thequencher is added to the information recording medium, the highreflectance can be maintained, and thereby the recorded information canbe reproduced by a commercially available CD player. Hence, an opticaldisc showing both of high reflectance and high resistance to light canbe also obtained.

Recording of information and reproduction of the recorded informationusing the above-described information recording medium can be made, forexample, in the following manner.

The information recording medium is irradiated with a light forrecording information (i.e., recording light) such as a semiconductorlaser beam from the substrate side under rotation of the medium at afixed linear speed (case of CD format signals: 1.2-1.4 m/sec) or a fixedangular speed. In the optical disc having a reflecting layer, it ispresumed that formation of cavities on the interface between therecording layer and the reflecting layer (this formation of cavities isaccompanied by distortion of any of the recording layer and thereflecting layer or both of them) which is apt to occur when thesublimation temperature of the dye is low, distortion of the substratewhich is apt to occur when the sublimation temperature of the dye ishigh, discoloration of the recording layer, variation of the recordinglayer in the association condition, etc., are brought about underirradiation with the recording light, and thereby the reflectance of theoptical disc is changed so as to record information on the optical disc.In the optical disc not having a reflecting layer, pits are generallyformed on the recording layer to record information. As the recordinglight, a semiconductor laser beam having a wavelength region of 750 to850 nm is generally employed.

Reproduction of the recorded information can be carried out byirradiating the information recording medium with a semiconductor laserbeam from the substrate side under rotation of the medium at the samefixed linear speed as described above and detecting the reflected light.

Examples of the present invention are given below, but those examples byno means restrict the invention. The examples include examples ofsynthesis of the compounds having the formulas (I) and (II) according tothe invention.

EXAMPLE 1 Synthesis of Compound I-4

4.58 g of 5,6-dichloro-1-ethyl-2-methylbenzimidazole was dissolved in 25ml of acetone. To the resulting solution was then added 3 ml ofα-bromopropiophenone and the resulting mixture was heated for 100minutes under reflux.

Acetone was removed from the mixture through distillation, and themixture was heated to 100° C. for 1 hour. To the mixture was then added100 ml of acetone and the mixture was stirred to produce colorlesscrystals. The colorless crystals were separated from the mixture throughfiltration. The crystals were washed with acetone and dried. A yield ofthe crystals was 5.4 g.

The crystals were added to an aqueous solution of 1.3 g of sodiumcarbonate in 60 ml of water, and the resulting mixture was heated to 80°C. on a steam bath.

The mixture was allowed to stand for cooling, and produced crystals wereseparated from the mixture through filtration. The crystals were thenwashed with water to obtain 4.8 g of colorless crystals. Thereafter, thecrystals were recrystallized from 600 ml of methanol, to obtain 2 g of6,7-dichloro-4-ethyl-1-methyl-2-phenylpyrrolo[1,2-a]benzimidazole in theform of colorless needle crystals.

1 g of the obtained crystals were dissolved in a mixture of 20 ml ofacetonitrile and 0.3 g of monohydrate of p-toluenesulfonic acid. To theresulting solution was added 1 ml of 1,3,3-trimethoxypropene, and theresulting mixture was heated for 5 minutes under reflux. The mixture wasallowed to stand for cooling, and produced crystals were separated fromthe mixture through filtration. The crystals were then washed with 25 mlof acetonitrile to obtain 0.75 g of crystals of Compound I-4 (meltingpoint: 247°-249° C.).

EXAMPLE 2 Synthesis of Compound 18

To 2.57 g of 5,6-dichloro-1-isopropyl-2-methylbenzimidazole were added1.52 ml of α-bromopropiophenone and 1 ml of anisole, and the mixture washeated for 2 hours on a steam bath. Then, to the mixture was added 50 mlof acetone. A produced crystals were separated from the mixture throughfiltration, then to the crystals were added 40 ml of 2% aqueous solutionof sodium carbonate, and the resulting mixture was heated for 80 minuteson a steam bath. After the mixture was allowed to stand for cooling,produced crystals were separated from through filtration. The crystalswere then washed with water to obtain6,7-dichloro-4-isopropyl-1-methyl-2-phenylpyrrolo[1,2-a]benzimidazole.

To 0.6 g of the crystals were added 11 ml of acetonitrile, 0.17 g ofmonohydrate of p-toluenesulfonic acid and 0.6 ml of1,3,3-trimethoxypropene, and the mixture were heated for 15 minutesunder reflux. After the resulting mixture was allowed to stand forcooling, produced crystals were separated from the mixture throughfiltration. The crystals were then washed with a small amount ofacetonitrile, to obtain 1.2 g of Compound I-18 (melting point: 167° C.).

Examples of the information recording medium according to the inventionand comparison examples are given below.

EXAMPLE 3

In 100 cc of 2,2,3,3-tetrafluoropropanol (formula: HCF₂ CF₂ CH₂ OH) weredissolved 2.0 g of the aforementioned cyanine dye (dye number: I-45) and0.2 g of a diimmonium dye (trade name: IRG-023, available from NipponKayaku Co., Ltd.) as a quencher, to prepare a coating solution for theformation of a dye recording layer.

On a disc-shaped polycarbonate substrate having been provided with atracking guide (outer diameter: 120 mm, inner diameter: 15 mm,thickness: 1.2 mm, track pitch: 1.6 μm, width of groove: 0.5 μm, depthof groove: 900 Å) was coated the above-obtained coating solution bymeans of spin coating at 1,000 r.p.m., and the substrate with a coatedlayer was then dried for 30 seconds to form a recording layer having athickness of 1,300 Å on the substrate.

Thus, an information recording medium consisting of a substrate and arecording layer was prepared.

EXAMPLE 4

The procedures of Example 3 were repeated except that a reflecting layerhaving a thickness of 1,300 Å was further formed on the recording layerby means of DC sputtering of Au and a protective layer having athickness of 3 μm was furthermore formed on the reflecting layer bycoating a UV-curable resin (trade name: 3070, available from Three BondCo., Ltd.) over the reflecting layer by means of spin coating at 1,500r.p.m. and curing the coated layer under irradiation with ultravioletrays by the use of a high-voltage mercury lamp.

Thus, an information recording medium was prepared.

EXAMPLE 5

The procedures of Example 4 were repeated except for using 2 g of theaforementioned cyanine dye (dye number: I-4) as a dye instead of 2 g ofthe cyanine dye I-17, to prepare an information recording medium.

EXAMPLE 6

The procedures of Example 5 were repeated except for using both 0.4 g ofthe cyanine dye I-4 and 1.6 g of the aforementioned cyanine dye (dyenumber: III-1) as a dye instead of 2 g of the cyanine dye I-4, toprepare an information recording medium.

EXAMPLE 7

The procedures of Example 6 were repeated except that a coating solutionfor the formation of a dye recording layer was prepared without addingthe diimmonium dye, to prepare an information recording medium.

EXAMPLE 8

The procedures of Example 6 were repeated except that a substrate havinga groove depth of 1,600 Å was used instead of the substrate having agroove depth of 900 Å, a coating solution for the formation of a dyerecording layer was prepared by varying the amount of the cyanine dyeI-4 from 0.4 g to 1.0 g and the amount of the cyanine dye III-1 from 1.6g to 1.0 g, and the thickness of the recording layer was varied from1,300 Å to 2,000 Å, to prepare an information recording medium.

EXAMPLE 9

The procedures of Example 6 were repeated except for using theaforementioned cyanine dye (dye number: III-6) instead of the cyaninedye III-1, to prepare an information recording medium.

EXAMPLE 10

The procedures of Example 6 were repeated except for using theaforementioned dye (dye number: I-18) instead of the cyanine dye I-4, toprepare an information recording medium.

COMPARISON EXAMPLE 1

The procedures of Example 7 were repeated except that a coating solutionfor the formation of a dye recording layer was prepared by varying theamount of the aforementioned dye III-1 from 1.6 g to 2.0 g without usingthe cyanine dye I-4, to prepare an information recording medium.

COMPARISON EXAMPLE 2

The procedures of Comparison Example 1 were repeated except that acoating solution for the formation of a dye recording layer was preparedby further adding 0.2 g of the above-mentioned diimmonium dye (tradename: IRG-023, available from Nippon Kayaku Co., Ltd.) as a quencher, toprepare an information recording medium.

The composition of each coating solution for the formation of a dyerecording layer obtained in the above examples and comparison examplesare set forth in Table 1.

                  TABLE 1                                                         ______________________________________                                                 Material [by weight]                                                          Dye I    Dye III     Quencher                                        ______________________________________                                        Example 3  I-45   [100]   --        IRG023 [10]                               Example 4  I-45   [100]   --        IRG023 [10]                               Example 5  I-4    [100]   --        IRG023 [10]                               Example 6  I-4    [20]    III-1 [80]                                                                              IRG023 [10]                               Example 7  I-4    [20]    III-1 [80]                                                                              --                                        Example 8  I-4    [50]    III-1 [50]                                                                              IRG023 [10]                               Example 9  I-4    [20]    III-6 [80]                                                                              --                                        Example 10 I-18   [20]    III-1 [80]                                                                              IRG023 [10]                               Com. Ex. 1 --         III-1 [100] --                                          Com. Ex. 2 --         III-1 [100] IRG023 [20]                                 ______________________________________                                    

Evaluation of Information Recording Medium

1) Reflectance

Each of the above-obtained information recording media was irradiatedwith a light having a wavelength of 780 nm from the substrate side tomeasure reflectance on the nonrecorded area of each medium using aspectrophotometer (produced by Hitachi, Ltd.).

2) C/N

Each of the above-obtained information recording media was irradiatedwith a semiconductor laser beam having a wavelength of 780 nm under theconditions of a fixed linear speed of 1.3 m/sec and a recording power of7 mW, to record a signal having a modulation frequency of 720 kHz (duty:33%) on each medium. The recorded signal was reproduced under thecondition of a reproducing power of 0.5 mW, to measure C/N of thereproduced signal using a spectrum analyzer (TR4135, produced byAdvantest Co., Ltd.).

The results are set forth in Table 2.

                  TABLE 2                                                         ______________________________________                                                      Reflectance                                                                           C/N                                                                   (%)     (dB)                                                    ______________________________________                                        Example 3       35        54                                                  Example 4       56        48                                                  Example 5       86        47                                                  Example 6       81        49                                                  Example 7       80        50                                                  Example 8       82        49                                                  Example 9       81        50                                                  Example 10      80        50                                                  Com. Ex. 1      73        50                                                  Com. Ex. 2      69        50                                                  ______________________________________                                    

As is evident from the results set forth in Table 2, the optical disc(Example 3) having a recording layer prepared using a cyanine dye of theinvention having absorption maximum on a relatively longer wavelengthside showed reflectance and C/N of high level. Thus, it has beenconfirmed that the cyanine dye can be satisfactorily employed as a dyefor optical discs. The optical disc (Example 4) having a reflectinglayer on the above-mentioned recording layer had a relatively highreflectance and was hardly reduced in C/N. The optical disc (Example 5)having a recording layer prepared using a dye of the invention havingabsorption maximum on a relatively shorter wavelength side and furtherhaving a reflecting layer on the recording layer had a markedly highreflectance and was hardly reduced in C/N. The optical discs (Examples 6to 10), each having a recording layer prepared using a combination of aspecific cyanine dye of the invention (e.g., cyanine dye used in Example5) and a dye having absorption maximum on the longer wavelength sidethan the wavelength of the cyanine dye, had a markedly high reflectanceand maintained C/N of high level. Accordingly, it was possible tomaintain a reflectance of high level even when a quencher was added tothe optical disc, as shown in the optical discs of Examples 5, 6, 8 and10.

On the other hand, the optical disc (Comparison Example 1) using only adye having benzoindolenine structure which is generally known as a dyehaving high reflectance and excellent recording and reproducingcharacteristics exhibited lower reflectance than the optical discs ofExamples, and such low reflectance was unsatisfactory in reproduction ofthe recorded information using a commercially available CD player.Therefore, in the case of the optical disc (Comparison Example 2)obtained by adding a quencher to the optical disc of Comparison Example1 for the purpose of improving resistance to light, the reflectance ofthe optical disc was lower than 70%.

Examples of the optical filter prepared by using the cyanine dyeaccording to the invention are given below.

EXAMPLE 11 Preparation of Optical Filter

0.1 g of the Compound I-4 was dissolved in 10 ml of2,2,3,3-tetrafluoro-1-propanol. The resulting solution was coated over aglass plate through spin coating to prepare an optical filter. Aabsorption spectrum of the optical filter measured by means of aspectrophotometer (Hitachi automatic spectrophotometer 340 type), hadthe absorption maximum at 708 nm. The transmittance of the opticalfilter at 780 nm was 59%.

From the results, it was confirmed that the optical filter was usefulfor cutting off near infrared rays.

EXAMPLES 12 & 13 Preparation of Solution-type Optical Filter

0.1 g of the Compound I-4 and 0.1 g of the Compound I-18 were eachdissolved in 100 ml of methanol, and each of the resulting solutions wasintroduced into a rectangular cell made of glass. Each of the solutionscontained in the cell was blue, and the absorption maximum of eachsolution was 626 nm. From the results, it was confirmed that thesolution-type optical filters were useful for cutting off a light havinga wavelength of approx. 626 nm.

We claim:
 1. An information recording medium comprising a substrate anda recording layer for recording information by means of a laser beamwhich is provided on the substrate, in which said recording layercontains a cyanine dye having the formula (I): ##STR16## wherein each ofA¹ and A¹⁰ is an atom group for completing an aromatic ring which mayhave one or more substituents; L¹ is a methine group which may have oneor more substituents or a trivalent connecting group of conjugated 3, 5or 7 methines which may have one or more substituents; each of R¹¹, R¹²,R¹³ and R¹⁴ is a hydrogen atom, or an alkyl, phenyl, acyl, alkoxy or 5-or 6-membered heterocyclic group which may have one or moresubstituents; each of R¹⁵ and R¹⁶ is an alkyl group which may have oneor more substituents and said one of substituents may have an anionicmoiety; X_(a) ^(p-) is an anion; p is 1, 2 or 3; m is 1 or 0; and m is 0when R¹⁵ or R¹⁶ has a substituent having an anionic moiety.
 2. Theinformation recording medium as claimed in claim 1, wherein each of A¹and A¹⁰, R¹¹ and R¹³, R¹² and R¹⁴, and R¹⁵ and R¹⁶ in the formula (I) isthe same as each other, each of A¹ and A¹⁰ is an atom group forcompleting a benzene ring which has one or more substituents or anaphthalene ring which may have one or more substituents, and each ofR¹¹, R¹³, R¹² and R¹⁴ is a hydrogen atom, an alkyl group which may haveone or more substituents or a phenyl group which may have one or moresubstituents.
 3. The information recording medium as claimed in claim 1,wherein a reflecting layer is provided on the recording layer.
 4. Theinformation recording medium as claimed in claim 1, wherein therecording layer comprises a mixture of a cyanine dye having the aboveformula (I) and a dye having absorption maximum on the longer wavelengthside than the absorption maximum of the cyanine dye.